On Tuesday, the National Transportation Safety Board (NTSB) determined that the two-train collision on the Massachusetts Bay Transportation Authority (MBTA) Green Line last year occurred as a result of the trolley operator's failure to obey a signal indication, likely because she became disengaged from her environment consistent with experiencing an episode of micro-sleep.

The lack of a positive train control system, which would have intervened to stop the train and prevented the collision, was cited as a contributing factor.

At 5:51 p.m. EDT, on May 28, 2008, an MBTA Green Line train, traveling westbound at about 38 mph, struck the rear of another westbound Green Line train, which had stopped for a red signal. The operator of the striking train was killed; three other crewmembers sustained minor injuries. Of the 185 to 200 passengers who were on the two trains at the time of the collision, four sustained minor injuries and one was seriously injured. Total damages were estimated to be about $8.6 million.

Post-accident toxicological testing indicated that the operator of the striking train had recently taken the drug doxylamine, commonly found in sleep aids, suggesting that she had trouble sleeping during at least one of the nights leading up to the accident. In addition, the operator had a high body mass index (BMI), which is closely correlated with a higher risk for sleep apnea, increasing the likelihood of a fatigued condition during waking hours.

"Again, we've seen a situation where a positive train control system could have prevented a tragic accident," said NTSB Acting Chairman Mark V. Rosenker. "We know that because operators or equipment sometimes fail, redundancies like PTC systems can be the difference between a fatal accident and an incident report. And this is why we feel so strongly that transit systems like MBTA's should have that crucial extra layer of safety that a PTC system provides."

Because the NTSB concluded that the accident could have been prevented had the MBTA been equipped with a positive train control system, the NTSB recommended to the Federal Transit Administration (FTA) that the agency facilitate the development and implementation of positive train control systems for rail transit systems nationwide.

In addition, the NTSB recommended to the FTA that they develop and disseminate guidance for operators, transit authorities and physicians regarding the identification and treatment of individuals at high risk for obstructive sleep apnea and other sleep disorders.

The NTSB recommended to MBTA that train operators be required to notify fellow crewmembers when a restrictive signal is encountered and the intended means of complying with the restriction.

A summary of the findings including the Board's report is available on the NTSB's Website at http://www.ntsb.gov/Publictn/2009/RAR0902.html

The entire accident docket is available at http://www.ntsb.gov/Dockets/Railroad/DCA08MR007/default.htm